The conventional wisdom about who will control the Senate next year has sure taken a turn. Republicans were once rated a safe bet to win a majority, and as late as September 2 were given an almost 50 percent chance of doing so by statistician Nate Silver on his New York Times blog.

Now the rough patches in Mitt Romney’s campaign have combined with some good breaks for Democrats to push Republican chances for a majority down to 21 percent, according to Silver’s latest calculations. Left-leaning bloggers are already proclaiming liberal favorites such as Elizabeth Warren in Massachusetts and Tammy Baldwin in Wisconsin to be winners. But their euphoria is overblown.

A more sober reading of the polls shows only that the key races are volatile. Control of the Senate will hinge on a dozen states where the lead is likely to swing back and forth for weeks while remaining within the margin of error. This uncertainty is partly because pollsters have begun to survey likely voters instead of registered voters. About 35 percent of registered voters won’t turn out in November, so pollsters must try to identify which voters will actually cast ballots. Variations between polls often result from differences in the predicted partisan breakdown of the electorate. It makes a big difference whether a pollster predicts that the 2012 electorate will resemble the electorate in 2008, when Democratic turnout was relatively high, or the electorate in 2010, which included a greater number of Republicans.

Take Massachusetts. Four polls in the last week showed Harvard law professor Elizabeth Warren running ahead of incumbent Scott Brown by between two and six points. But a University of Massachusetts/Boston Herald poll released Thursday shows Brown up 50 percent to 44 percent among registered voters and with a four-point lead among likely voters. The difference between the Herald poll and, say, a Public Policy Polling survey that shows Warren with a two-point lead appears to be the partisan breakdown of likely voters who were surveyed. Self-described Democrats made up 28 percent of the Herald survey and 38 percent of the PPP respondents (independents are the largest share of the state’s electorate). But both polls show that an increasing number of voters think Warren is “too liberal” even for Massachusetts.

In nearby Maine, former independent governor Angus King had held a commanding lead in the race to replace retiring Republican Olympia Snowe, but two polls released Wednesday show King losing support in the three-person race.

PPP’s survey has King at 43 percent, with Republican secretary of state Charles Summers closing in at a surprising 35 percent and Democrat Cynthia Dill at 14 percent. A poll from the left-leaning Maine People’s Resource Center has King at 44 percent, Summers at 28 percent, and Dill at 15 percent. Both polls confirm that support for Dill has grown substantially. This surge may be due in part to a group with Republican ties called Maine Freedom, which has run ads encouraging Democrats to vote for Dill instead of King. Republicans believe that if they can help push Dill’s support up to about 20 percent, a victory for Summers is possible. At the same time, attacks on King — who has endorsed President Obama for reelection — have taken their toll on his GOP support. The percentage of Republicans in the PPP poll who view King negatively has increased to 65 percent from just 38 percent in March.

“The Maine Senate race is closer now than anyone really expected it to be,” PPP president Dean Debnam said recently. “Angus King is losing more Democrats than he is picking up Republicans, and although he remains the favorite, a victory for him is not as inevitable as it used to be.”

Republicans are showing signs of life in some unexpected places. In Connecticut, GOP candidate Linda McMahon has outspent Democrat Chris Murphy by seven to one and is currently ahead. In Missouri, it looks as if Republican Todd Akin will stubbornly stay in the race. Akin, despite his horrendous gaffe about pregnancy and rape in August, has moved back to within striking distance of unpopular Democratic incumbent Claire McCaskill, and he has convinced Newt Gingrich to come to St. Louis for a fundraiser next week. Republican strategists warn, however, that Akin still faces a tsunami of negative ads in the home stretch.

Democrats are also outperforming expectations in some Senate races. North Dakota features colorless GOP congressman Rick Berg against former Democratic state attorney general Heidi Heitkamp. Berg has failed to unite his party behind him after defeating a conservative challenger in the primary. In Indiana the battle between GOP nominee Richard Mourdock and Democratic congressman Joe Donnelly is very close. But Republicans believe Donnelly has not yet been “defined” in the electorate’s mind, and they plan to define him with a flood of outside advertising.

Here is a rundown of other crucial Senate races based on my conversations with key observers:

Florida. Democratic senator Bill Nelson is pounding Republican congressman Connie Mack and has opened up a lead. But this is a must-win state for Romney, and massive Republican spending in it will give Mack a chance to come from behind.

Nevada. GOP incumbent Dean Heller, who was appointed to the Senate after John Ensign’s resignation, has finally taken a lead over Democratic congresswoman Shelley Berkley. But the race will remain in flux because Senate majority leader Harry Reid is spending a lot of money on it and the Nevada Democrats have a formidable turnout operation.

Hawaii. Hawaii leans to the left, and any race in a state where Barack Obama is seen as a hometown hero is tough for Republicans, but Democratic congresswoman Mazie Hirono is a plodding candidate. She managed to lose a race for governor in 2002 to Republican Linda Lingle, who is her opponent for the Senate this year. Internal polls show Hirono holding a seven-point, but Lingle has a chance.

Montana. Democratic incumbent Jon Tester is running a personality campaign to overcome the conservative tilt of his state. Republican congressman Denny Rehberg is attempting to tie Tester to Obama. In the latest polls, Rehberg is up by a single point. This race will be close to the end.

New Mexico. In a state where Hispanics are 40 percent of the population, Republicans face tough sledding. Former GOP congresswoman Heather Wilson is down by five points in her internal polling, and by eight points in public polls. She is a clear underdog but could perhaps crawl back.

Ohio. Mitt Romney’s struggles in Ohio have been reflected in the Republican Senate candidate’s poll numbers. Mandel is down four points, but it remains a competitive race.

Virginia. A new Washington Post poll shows Democrat Tim Kaine with an eight-point lead over Republican George Allen, but the poll’s partisan breakdown may be too heavily skewed towards Democrats. Mitt Romney trails in Virginia, but it’s possible Allen could win even if Republicans lose the state on the presidential level. He has gained traction by arguing that looming defense cuts — imposed by last year’s Budget Control Act — will be devastating for Virginia.

Wisconsin. New polls show that Democrat Tammy Baldwin has opened up a lead over former governor Tommy Thompson, whom she has attacked for his lobbying work in Washington. But Baldwin, currently a representative in the U.S. House, has a voting record that puts her far to the left of most of Wisconsin’s electorate, and she hasn’t yet been hit much on it. Republicans expect to pick up this seat, and Democrats are reluctant to discuss their prospects.

What is clear from reading all of these tea leaves is that there are almost a dozen Senate races in play. It’s entirely possible that many will be close enough to cause recounts or even court battles, potentially leaving control of the Senate up in the air for days or weeks after the election.